1. Field of the Invention
The present invention relates to the selective production of C6–C10 hydrocarbons useful as automotive fuel components. In particular, the present invention concerns a process for dimerizing lower, olefinic hydrocarbons in the presence of a zeolite catalyst under process conditions allowing selective dimerization.
2. Description of Related Art
Light olefin dimers are useful intermediates in the manufacture of different products, like alcohols, ketones and carboxylic acids. Highly branched trimethylolefins and trimethylparaffins are useful as gasoline octane number enhancers.
Zeolites represent a diverse family of crystalline microporous materials with a wide variety of topologies, chemical compositions and acid-base properties. It is commonly known that zeolites are commercially used as catalysts in several different hydrocarbon transformation reactions, such as cracking and isomerization.
Many processes for utilization of light olefins for the production of high quality transportation fuels are known. The Mobil Olefin to Gasoline and Distillate (MOGD) process converts propylene and butylene to olefinic distillate in high yields. The MOG or Mobil Olefins to Gasoline process is an extension of the MOGD. In MOG, the reaction conditions allow aromatics formation. Oligomerization of isobutene from C4 olefins over zeolite catalysts has been disclosed in several U.S. patents.
An essential feature of selective dimerization of light olefins is to prevent oligomerization, which is the successive reaction of dimerization. Selective dimerization of isobutene over trimerization and higher oligomerization is known from U.S. Pat. No. 3,325,465, DE Patent No. 3 542 171 and International Patent Application WO 01/46095. In U.S. Pat. No. 3,325,465, the use of nickel and cobalt ions in 13× zeolite is disclosed. In DE Patent No. 3 542 171, the selective dimerization of isobutene into trimethylpentene is made with bismuth or lead doped zeolite. In WO 01/46095, large pore zeolite Beta has been found to be selective for isobutene dimerization to the trimethylpentene in the presence of oxygenates. In addition, the reference teaches that dimerization can be carried out in the presence of an alcohol.
There are a few commercial dimerization processes. Institut Francais du Petrole (IFP) has developed a process for dimerization of light olefins (Dimersol). The Octol process, developed by UOP and Huls AG, produces linear octenes, which are the raw-materials of plastics softeners. Homogeneous catalysts are used in the processes.
There are several processes based on ion exchange resins available, as described e.g. in U.S. Pat. Nos. 4,375,576 and 4,100,220. They have many good properties but they all have the drawback of being totally dependant on oxygenate moderator, which improves the selectivity. This moderator has to be recycled and there is usually the same amount of oxygen containing side products present in the dimerized product. When the reason to use dimerization is elimination of oxygenates from the components, these are highly undesired. Moreover, the oxygenates make hydrogenation of dimerized product more difficult.
All problems connected to oxygenates are self-evidently solved if they can be removed from the process altogether.
As the above review of the art shows, none of the known solutions provides dimerization at high selectivity in combination with facile separation of unreacted components. There is a need for a process that operates with heterogeneous catalysts at industrially operable conditions without polar compounds.